1. Field of the Invention
This invention relates to a process of converting polynuclear aromatic compounds to mononuclear aromatic compounds. More particularly, this invention is directed to the oxidation of polynuclear aromatic compounds such as anthracene to produce mononuclear aromatic compounds such as benzene.
This invention is also directed to a process for cracking the center ring of polynuclear aromatic compounds.
2. Description of the Prior Art
Polynuclear aromatic compounds such as anthracene, anthracene derivatives, fluorene and fluorene derivatives, and particularly anthracene, have long been valuable items of commerce. These materials are primarily used for preparing oxidized products such as anthraquinone, which is a valuable intermediate in the preparation of stable dyes for the dye industry. While the polynuclear aromatic compounds and the oxidized derivatives thereof can be synthesized from various materials, such synthesis is a very costly source of these compounds. As a result, the primary source of such materials is coal liquids produced by the solvent extraction or destructive dissolution of carbonaceous materials, particularly coal. However, one of the difficulties of recovering these materials from coal liquids is that they are present in such minute quantities that they are difficult to separate from one another and from contaminating materials of substantially similar properties and boiling points.
Although a number of techniques have been suggested for separating anthracene and fluorene from coal liquids, such separations are extremely complex and expensive. This is due to the fact that processes such as fractional crystallization, solvent extraction, and the like, which are expensive, must be used because of the presence of contaminating material in the fraction which reacts and boils similarly.
Additionally, there is a number of processes in the prior art for converting condensed-ring aromatics to their oxidized derivatives, in many cases without separating these compounds from their crude mixtures. However, many of these processes have features which limit their application on a commercial scale.
The conversion of anthracene to anthraquinone in the presence of a catalyst is taught by Ho et al., Synthesis, 10,560 (1972). According to this reference, polynuclear aromatic hydrocarbons, such as anthracene, can be oxidized to form 9, 10-anthraquinone, using ceric ammonium nitrate as a catalyst. The reaction is carried out at a temperature of 25.degree. C. This process suffers from the disadvantage that a large amount of the catalyst is consumed as the reaction proceeds, making it necessary to replenish the catalyst supply and thus increasing the cost of the product formed.
The thermal decarbonylation of anthraquinone to biphenyl and then benzene is disclosed by Sakai et al, Chemistry Letters, pp 617-620 (1974). According to this reference, the following reaction scheme is taught: ##STR1## The reaction is conducted in the presence of hydrogen at a temperature from about 800.degree. to 1110.degree. F. The presence of catalysts is not required according to this reference.
The present inventor has discovered that in the oxidation of anthracene to 9, 10-anthraquinone, the catalytic reaction may be substantially improved by conducting the reaction at a temperature of from about 65.degree. to about 205.degree. C. (149.degree.-401.degree. F.) and in the presence of a promoter such as potassium chlorate (KCl O.sub.3) and in the absence of water in the reaction system. Under these conditions, the number of moles of anthracene converted per mole of catalyst used is increased several times.
Furthermore, it has also been discovered that by using a catalyst such as zeolite, 9, 10-anthraquinone can be converted to benzene at a temperature of about 425.degree. C. (797.degree. F.) or above without using hydrogen as a reactant. This discovery is significant in that hydrogen is becoming an increasingly expensive feed material. By eliminating hydrogen as a reactant in the conversion of anthraquinone to benzene, the cost of this process is substantially decreased.